This invention relates to impact-resistant polyamide molding compounds.
Polyamides are known and proven construction materials which can be processed, e.g., by injection molding or extrusion processes. In general, polyamides, especially after conditioning, exhibit good toughness. However, for certain uses, improvements with respect to impact strength and notch impact strength in the injection fresh condition (i.e., immediately after injection molding) and especially at low temperatures (e.g. down to -40.degree. C.) are desirable.
A suitable way of approaching this goal is the use of polyamide blends, i.e. intimate mixtures of polyamides with tough elastomers or tough, high-molecular weight thermoplastics. Special primary properties of these added polymers can thus be transferred to the polyamide blends without destroying the typical polyamide properties.
To obtain polyamide blends with satisfactory properties in each case the polymers to be mixed in are optimized for the specific use and the respective polyamide type employed in the blend. Since such special polymers are not available on the market in the necessary variety, as a rule there is the problem that in each case special products in mostly small amounts must be produced in a cost-intensive way.
Because of these problems, in practice an alternative approach is taken in the production of polyamide-blend polymers which requires the use of olefinic polymers, available in large amounts and varieties, e.g., polyethylene or ethylene/proplene/diene copolymers, which exhibit a high cold or notch toughness.
According to DE-PS 11 31 883, mixtures of (1) polycondensates containing carbonamide groups; (2) polyolefins, polystyrene, olefin or styrene copolymers, which, besides olefin or styrene, can also contain other olefinic unsaturated monomers incorporated by polymerization; and (3) catalysts forming radicals are homogenized at 50.degree.to 350.degree. C., whereby graft copolymers are believed to form from (1) and (2). However, the products thus obtained are not completely satisfactory.
The obviously preferred and most effective embodiment of the alternative approach discussed above consists of the grafting of high-molecular weight olefinic elastomers preferably with, e.g., maleic anhydride, according to the working processes described in DE-OS 24 01 149. However, such processes have the disadvantage that because of the action of maleic anhydride, which is greatly hazardous to health, the reaction cannot easily be performed in the usual industrial installations. Moreover, the efficient elastomers are difficult to handle because of their tackiness, and because necessary machines develope a high shear power and due to the thermal stress involved, the graft process often results in damages to the elastomers by discoloration, decomposition and/or partial cross-linking. Polyamide blends with such added polymers are described in DE-OS 26 22 973.
All the products that can be used for the alternative method described above must exhibit a high molecular weight and consequently have high melting viscosities, if they are to produce good results in the polyamide blends. At the same time optimal properties and efficiency are attained only if the added polymers are finely dispersed in the polyamide matrix. This incorporation causes difficulties because of the high viscosity of the added polymers and the relatively low viscosity of the polyamides and the thus unfavorable polyamide/added polymer viscosity relation. Fluctuations in the flow rate and shearing conditions make the production of a reproducibly good quality difficult. Use of the mixing aggregates possessing a high shear power does reduce the scattering of the values but requires a high energy expenditure and can result in damage to the polyamide blends.